High purity steam is used in many applications in current advanced technology processes, including processes employed in semiconductor manufacturing, production of medical gases, and fuel cell technology. Because high purity steam introduces minimal impurities, such as toxic waste byproducts, into a process, it can be used, e.g., for the oxidation of silicon, in the production of thin gate oxides (e.g., in the electronics and semiconductor industries), and in ultra-high purity cleaning processes.
In the pharmaceutical and biotechnology industries, high purity steam is used for sterilization, or can be condensed to yield high purity water. Although generally more expensive than standard de-ionization processes, the production of liquid water from high purity steam can yield a product having reduced amounts of, e.g., prions, viruses, allergens, proteins, bacteria, and other biologically active macromolecules or substances present in biological systems that may not be effectively removed by standard water de-ionization processes. Additionally water containing substantially reduced levels of inorganic substances, such as borates and silicates that commonly pass through de-ionized water systems, or metallic substances such as iron, nickel, chrome, copper, and other toxic metals characteristic of water produced from metal stills, can be obtained from ultrapure steam.
Typically, steam for technological and industrial applications is produced by simply boiling de-ionized water or by reacting gaseous hydrogen and oxygen to yield water vapor. In the latter case, the production of pure steam is practically impossible due to the presence of residual oxygen and/or hydrogen remaining in the product water vapor. Removing these components often requires additional expensive and complex separation processes. Additionally, high concentrations of gaseous hydrogen are often required for the synthesis reaction with oxygen, which is conducted at high temperatures well above the explosive limit of hydrogen (approximately 8% at a pressure of approximately 100 kPa). Steam synthesis processes operated under such conditions can present dangerous safety problems if not properly conducted.
The simple boiling of high purity de-ionized water to yield steam can avoid the problems and dangers inherent in the direct reaction of hydrogen and oxygen to yield steam. However, removing dissolved gases can be difficult and often requires multiple boiling/condensation cycles in a hermetically sealed environment, which can be expensive. Moreover, aerosols containing materials that are not normally volatile, such as salts or metals, can be produced during the boiling process. When steam containing such aerosols is condensed at the point of use, these impurities may be incorporated into the condensate and can add unwanted impurities to the liquid water, and therefore, higher costs due to subsequent process steps required for the removal of the impurities. Because ultrapure water itself is very corrosive, whatever material is used to construct the boiler (e.g., quartz, stainless steel, glass, etc.) can be dissolved into the steam and then entrained in aerosols.